1. Technical Field
The present invention relates to reticles for semiconductor processing in general, and in particular to a method and apparatus for verifying stitching accuracy of stitched chips on a wafer.
2. Description of Related Art
During a typical chip fabrication, a field in a reticle is repeatedly exposed on different areas of a wafer until the entire photoresist layer has been exposed on the wafer. A stepper device is commonly utilized to move the reticle between exposures on the wafer.
The field of exposure is defined as the photoresist area exposed through one field at a time. Conventional photolithography equipment has a relatively small field of exposure. However, the desirable chip size of an image sensor tends to far exceed the limited field of exposure of conventional photolithography equipment. Thus, the size of a chip that can be manufactured on a single wafer is limited by the field of exposure of the photolithography equipment used to pattern the interconnect structures of the chip.
In order to make a larger chip on a single wafer, it is often necessary to stitch exposure fields together during the photolithography process. The accuracy of the stitching overlaps basically dictates how well the larger stitched chip will function. Because optical methods of verifying stitching accuracy can be somewhat subjective, it would be desirable to provide an improved method and apparatus for verifying stitching accuracy of a stitched chip on a single wafer.